1. Field of the Invention
This invention relates to a membrane electrode assembly and a fuel cell unit. In particular, the invention relates to a membrane electrode assembly and a fuel cell unit that are formed such that an outer peripheral portion of a solid polymer electrolyte membrane extends from an anode electrode and a cathode electrode on both sides thereof.
2. Description of the Related Art
A fuel cell unit, being a basic unit of a fuel cell, is constructed such that a membrane electrode assembly is sandwiched on both sides between a separator provided with fuel gas flow passages and a separator provided with oxidant gas flow passages. The membrane electrode assembly is constructed such that a pair of electrodes is installed, on each side of a solid polymer electrolyte membrane, and each of the pair of electrodes is provided with an electrode catalytic layer and a gas diffusion layer. Accordingly, in the electrode (anode electrode) to which fuel gas (for example, hydrogen) is supplied from the fuel gas flow passages, hydrogen is ionized, and is transferred to the other electrode (cathode electrode) via the solid polymer electrolyte membrane. Electrons generated during this time are extracted to an external circuit, and used as direct current electric energy. In the cathode electrode, to which oxidant gas (for example, air containing oxygen) is supplied, hydrogen ions, electrons and oxygen react, and water is generated.
Such membrane electrode assemblies are disclosed in Japanese Unexamined Patent Application, First Publication No. 5-234606, Japanese Unexamined Patent Application, First Publication No. 6-96783, Japanese Unexamined Patent Application, First Publication No. 8-148169 and U.S. patent application Ser. No. 5,464,700.
Typically, the outer peripheral portion of a solid polymer electrolyte membrane extends from the electrodes on both sides in order to function as a seal for preventing the mixing of fuel gas on the anode electrode side and oxidant gas on the cathode electrode side. However, since the solid polymer electrolyte membrane itself is very thin, the extending portion from the electrodes can be easily damaged by the pressure difference (pressure difference between electrodes) between the fuel gas and the oxidant gas mentioned above. In order to prevent such damage, consideration has been given to reinforcing the portion that extends in the membrane electrode assembly (extending part).
Japanese Unexamined Patent Application, First Publication No. 5-234606 discloses a membrane electrode assembly which is provided with a stiffening member in a frame shape contacting an extending part of a solid polymer electrolyte membrane. However, since there is a gap between the stiffening member and electrode, there is a possibility of a difference in moisture content between the gap and the other areas caused by generated water being deposited in this gap, or the like. If the moisture content differs, it causes a difference in extension and contraction force, and stress occurs in the gap. Therefore, there is a problem in that the solid polymer electrolyte membrane may be damaged. Furthermore, when forming a fuel cell unit using a membrane electrode assembly, it is necessary to provide a sealing structure in order to prevent fuel gas or the like from leaking outside. However, Japanese Unexamined Patent Application, First Publication No. 5-234606 does not disclose a sealing structure in the membrane electrode assembly, so there is a problem in that a sealing structure is complicated due to the provision of the stiffening member.
Japanese Unexamined Patent Application, First Publication No. 6-96783 discloses a membrane electrode assembly wherein frame-shaped gaskets are provided on both sides of a solid polymer electrolyte membrane. However, in this construction, there are problems in that by providing gaskets on both sides of the membrane, the number of parts increases, the routing becomes complicated, and cost becomes high.
In the membrane electrode assembly disclosed in Japanese Unexamined Patent Application, First Publication No. 8-148169, the solid polymer electrolyte membrane extends to the outer periphery. Therefore, proportionately more, expensive, solid polymer electrolyte membrane material is required, so there is a problem in that it is expensive.
The membrane electrode assembly disclosed in U.S. Pat. No. 5,464,700 requires that a seal and electrode overlap. Therefore, when the membrane electrode assembly is laminated and pressurized, the load is concentrated on the overlapping part, while the other areas do not have sufficient pressure, so there is a problem of possible instability.